Introduction

Creating a Generic Repository pattern in an MVC3 application with Entity Framework is the last topic that we are about to cover in our journey of learning MVC.

The article will focus on Unit of Work Pattern and Repository Pattern, and shows how to perform CRUD operations in an MVC application when there could be a possibility of creating more than one repository class. To overcome this possibility and overhead, we make a Generic Repository class for all other repositories and implement a Unit of Work pattern to provide abstraction.

Our roadmap towards Learning MVC

Just to remind our full roadmap towards learning MVC,

Pre-requisites

There are few pre-requisites before we start with the article,

We have running sample application that we created in fifth part of the article series.

We have Entity Framework 4.1 package or DLL on our local file system.

We understand how MVC application is created (follow second part of the series).

Why Generic Repository

We have already discussed what Repository Pattern is and why do we need Repository Pattern in our last article. We created a User Repository for performing CRUD operations, but think of the scenario where we need 10 such repositories.

Are we going to create these classes? Not good, it results in a lot of redundant code. So to overcome this situation we’ll create a Generic Repository class that will be called by a property to create a new repository thus we do not result in lot of classes and also escape redundant code too. Moreover we save a lot of time that could be wasted creating those classes.

Unit of Work Pattern

According to Martin Fowler Unit of Work Pattern “Maintains a list of objects affected by a business transaction and coordinates the writing out of changes and the resolution of concurrency problems."

From MSDN, The Unit of Work pattern isn't necessarily something that you will explicitly build yourself, but the pattern shows up in almost every persistence tool. The ITransaction interface in NHibernate, the DataContext class in LINQ to SQL, and the ObjectContext class in the Entity Framework are all examples of a Unit of Work. For that matter, the venerable DataSet can be used as a Unit of Work.

Other times, you may want to write your own application-specific Unit of Work interface or class that wraps the inner Unit of Work from your persistence tool. You may do this for a number of reasons. You might want to add application-specific logging, tracing, or error handling to transaction management. Perhaps you want to encapsulate the specifics of your persistence tooling from the rest of the application. You might want this extra encapsulation to make it easier to swap out persistence technologies later. Or you might want to promote testability in your system. Many of the built-in Unit of Work implementations from common persistence tools are difficult to deal with in automated unit testing scenarios."

The Unit of Work class can have methods to mark entities as modified, newly created, or deleted. The Unit of Work will also have methods to commit or roll back all of the changes as well.

The important responsibilities of Unit of Work are,

To manage transactions.

To order the database inserts, deletes, and updates.

To prevent duplicate updates. Inside a single usage of a Unit of Work object, different parts of the code may mark the same Invoice object as changed, but the Unit of Work class will only issue a single UPDATE command to the database.

The value of using a Unit of Work pattern is to free the rest of our code from these concerns so that you can otherwise concentrate on business logic.

Why use Unit of Work?

Again Martin Fowler statements, "When you're pulling data in and out of a database, it's important to keep track of what you've changed; otherwise, that data won't be written back into the database. Similarly you have to insert new objects you create and remove any objects you delete.

You can change the database with each change to your object model, but this can lead to lots of very small database calls, which ends up being very slow. Furthermore it requires you to have a transaction open for the whole interaction, which is impractical if you have a business transaction that spans multiple requests. The situation is even worse if you need to keep track of the objects you've read so you can avoid inconsistent reads.

A Unit of Work keeps track and takes responsibility of everything you do during a business transaction that can affect the database. When you're done, it figures out everything that needs to be done to alter the database as a result of your work."

You see I don’t have to concentrate much on theory, we already have great definitions existing, all we needed is to stack them in a correct format.

Using the Unit of Work

One of the best ways to use the Unit of Work pattern is to allow disparate classes and services to take part in a single logical transaction. The key point here is that you want the disparate classes and services to remain ignorant of each other while being able to enlist in a single transaction. Traditionally, you've been able to do this by using transaction coordinators like MTS/COM+ or the newer System.Transactions namespace. Personally, I prefer using the Unit of Work pattern to allow unrelated classes and services to take part in a logical transaction because I think it makes the code more explicit, easier to understand, and simpler to unit test(From MSDN).

Creating a Generic Repository

Cut the Redundancy…

Step 1: Open up our existing MVC3 application created in Part5 in Visual Studio.

Step2: Right click Learning MVC project folder and create a folder named GenericRepository and add a class named GenericRepository.cs to that folder.

We can see, we have created the generic methods and the class as well is generic, when instantiating this class we can pass any model on which the class will work as a repository and serve the purpose.

TEntity is any model/domain/entity class. MVCEntities is our DBContext as discussed in earlier parts.

Step 3: Implementing UnitOfWork: Create a folder named UnitOfWork under LearningMVC project, and add a class UnitOfWork.cs to that folder.

We see the class implements IDisposable interface for objects of this class to be disposed.

We create object of DBContext in this class, note that earlier it was used to be passed in Repository class from a controller.

Now it's time to create our User Repository. We see in the code itself that, simply a variable named userRepository is declared as private GenericRepository<User> userRepository; of type GenericRepository serving User entity to TEntity template.

Then a property is created for the same userRepository variable in a very simplified manner,

You see it was as simple as that, you can create as many repositories you want by just creating simple properties, and no need to create separate classes. And now you can complete the rest of the story by yourself, confused???? Yes it's DBOperations, let's do it.

Now this unitOfWork instance of UnitOfWork class holds all th repository properties,if we press “." After it, it will show the repositories.So we can choose any of the repositories created and perform CRUD operations on them.

Conclusion

We now know how to make generic repositories too, and perform CRUD operations using it.

We have also learnt UnitOfWork pattern in detail. Now you are qualified and confident enough to apply these concepts in your enterprise applications. This was the last part of this MVC series, let me know if you feel to discuss any topic in particular or we can also start any other series as well.